Relay Coordination Technical Guide

1. Protection coordination and selectivity

Relay coordination checks the protective-device response for credible fault locations and operating conditions. The closest suitable device should normally clear the fault first, with coordinated upstream backup where required by the protection philosophy.

A meaningful study evaluates pickup levels, calculated operating times, maximum and minimum fault currents, load and motor starting, transformer ratios and earthing, equipment withstand and the required grading margin.

2. Inverse-time and definite-time characteristics

For a typical IEC inverse-time function, operating time can be expressed as t = TMS × k / [(I / Is)^α − 1] for currents above the applicable pickup, where Is is the current setting. The coefficients depend on the selected curve family, and actual relay settings must follow the manufacturer's implementation and the applicable standard.

The RelayCoordination.com study model uses its configured composite characteristic: IDMT below the definite-time pickup, then definite-time operation after pickup. It does not continue an independent IDMT tail beyond the definite-time pickup for that modeled relay.

3. Earth-fault coordination across transformers

Earth-fault current paths depend on transformer winding arrangement, the available neutral/earthing connection and the studied fault side. Do not treat line current as zero-sequence relay current without checking the winding connection and measurement arrangement.

Coordination should explicitly account for whether a downstream earth-fault path exists and which upstream phase or earth-fault protection elements can detect a particular downstream fault.

4. Time-current curves and operating margins

Plot the downstream primary and upstream backup protective devices on the same current basis where appropriate. Determine their operating times at common fault points, with any transformer-side current conversion. Check the separation against the study's required margin.

Review minimum and maximum fault levels and confirm that apparent coordination is not an artifact of extrapolating an inactive relay section outside its applicable current range.

5. Motor starting and load limits

Compare the motor-starting characteristic, applicable base load, thermal limits and the protection TCC curves. A protection scheme must satisfy both fault-clearing requirements and acceptable starting/load operation.

6. Validation checklist

• Confirm the latest approved single-line diagram, device ratings and CT/VT ratios.
• Use credible minimum and maximum short-circuit and earth-fault levels.
• Verify the selected inverse-time family and definite-time settings against manufacturer data.
• Check transformer connections and earth-fault paths for every fault zone.
• Check grading margins and cross-overs at all relevant currents.
• Review motor starting, equipment thermal withstand and applicable fault-current limits.
• Validate final settings by qualified engineering review and commissioning procedures.